The present invention relates to ceramic matrix composite material parts. More particularly, it relates to improving the surface state of such parts.
In aeroengines, and in particular in the gas turbines or turbomachines of such engines, parts that present aerodynamic shapes, such as blades, are usually made of metal alloys using a method of casting and local machining. Present and future requirements concerning reducing specific fuel consumption, diminishing pollution, etc, in aeroengines are leading to a significant increase in their weight, and in particular in the low-pressure stages of the turbines.
Blades constitute an important portion of the weight of low-pressure stages. In order to significantly reduce weight and accommodate operating temperatures higher than those possible with present-day metal alloys, one solution would be to use ceramic matrix composite materials for making blades.
Ceramic matrix composite (CMC) materials are particular so-called “thermostructural” composite materials, i.e. composite materials having good mechanical properties and the ability to conserve those properties at high temperature. Furthermore, parts, such as blades, that are made of CMC present a significant weight saving compared with the same parts made using the usual metal alloys.
In well-known manner, CMC parts are formed by fiber reinforcement made of refractory fibers (carbon fibers or ceramic fibers), which reinforcement is densified by a ceramic matrix, in particular a matrix of carbide, nitride, refractory oxide, . . . Typical examples of CMC materials are C—SiC materials (carbon fiber reinforcement and silicon carbide matrix), SiC—SiC materials, and C—C/SiC materials (matrix comprising a mixture of carbon and silicon carbide). The fabrication of CMC composite parts is well-known. The fiber reinforcement may be densified using a liquid technique (impregnation with a ceramic matrix precursor resin and transformation of the resin into ceramic by curing and pyrolysis, which process may be repeated), or by a gas technique (chemical vapor infiltration).
Nevertheless, CMC parts present a surface appearance that is undulating and relatively rough, which may be found to be incompatible with the aerodynamic performance required of parts such as blades. Surface undulations are due to the fiber reinforcement, whereas roughness is associated with the “seal-coat” ceramic matrix, in particular when the matrix is deposited by chemical vapor infiltration (CVI).
Conversely, parts made of metal alloys and the associated methods present a surface aspect that is smooth, with roughness that is very small (of the order of 1 micrometer (μm)).
One solution for improving the surface state of a CMC part consists in applying a liquid composition to the surface of the part, the composition containing a ceramic-precursor polymer, e.g. a precursor of silicon carbide, together with a refractory solid filler in the form of grains serving to form a ceramic coating. The ceramic coating serves to level out the undulations present at the surface of the part. That step is followed by depositing a ceramic, e.g. SiC, performed by chemical vapor infiltration for a duration of about 30 hours (h) approximately, thereby bonding together the grains of the refractory filler. Such a surface treatment method for a CMC part is described in document US 2006/0141154.
Although that method makes it possible significantly to improve the surface state of a CMC part by eliminating the undulations and by reducing surface roughness to values of less than 40 μm, the need for additional chemical vapor infiltration after the ceramic coating has been formed gives rise to considerable increases in the cost of the part and in the time taken to fabricate it.